In U.S. Pat. No. 3,873,804, a welder's helmet is disclosed which uses as a protective lens assembly a liquid crystal light shutter. Such a light shutter comprises a layer of nematic liquid crystal material sandwiched between parallel transparent plates coated on their faces in contact with the liquid crystal material with transparent conductive coatings. By rubbing the transparent conductive coating in contact with the liquid crystal material at right angles to each other, a twisted nematic structure results which will rotate the plane of polarized light through 90.degree.. However, when an electrical field is applied across the liquid crystal material, the plane of polarization will no longer be rotated through 90.degree.. Thus, if crossed polarizers are disposed on opposite sides of the liquid crystal cell and their planes of polarization are parallel to the respective rubbed lines on the transparent coatings, polarized light will normally pass through the cell. However, when an electrical field is applied across the liquid crystal material and the plane of polarization is no longer rotated through 90.degree., the cell will block light. Alternatively, by using parallel polarizers, the cell will normally be opaque, or substantially opaque, and will become light-transmitting only when an electrical field is applied across the liquid crystal material.
While most applications for flash protection require a light transmission of only 0.01%, a liquid crystal lens assembly of the type described above will achieve a minimum light transmission of about 0.1% due to the fact that the liquid crystal material is a semi-ordered fluid which will scatter and depolarize some of the light passing therethrough.